We determined the rate of quadriceps muscle protein synthesis using the in vivo rate of incorporation of intravenously infused [13C]leucine into mixed-muscle protein in young , late middle-aged , and old women and men before and after 2 wk of resistance exercise training. The findings indicate that, although the initial quadriceps muscle protein synthesis rate was lower in late middle-aged and old than in young adults, a marked increase occurred in all age groups without an increase in the rate of whole body protein breakdown.